1. Field of the Invention
The present invention relates to a battery protection device for protecting a rechargeable battery, such as a lithium-ion rechargeable battery, from overcharging and overdischarging.
2. Description of the Related Art
Charging or discharging of rechargeable batteries beyond their suitable charge or discharge conditions may cause breakdown or deterioration of the battery. In particular, lithium-based rechargeable batteries tend to fail due to generation of gas and temperature increase when the electrolyte dissolves because of continuing overcharging. There is thus a need for a charge control, which makes sure that the rechargeable battery is not subjected to overcharging. It is therefore the normal practice to provide a battery protection device and to control charging and discharging of the rechargeable battery through the battery protection device, wherein the charge/discharge circuit is interrupted when overcharge or over-discharge is detected.
FIG. 6 shows the configuration of a protective device for a rechargeable battery as disclosed in Japanese Patent No. 2872365. A first switching element 31 and a second switching element 32 are connected in series in a charge/discharge circuit that connects a rechargeable battery 30 with input/output terminals 34 and 35. A control means 33 switches the first and the second switching element 31 and 32 between a conducting state and an interrupting state. The control means 33 detects the voltage of the rechargeable battery 30, in accordance with which it turns the first switching element 31 into the conducting state when the battery voltage is below a charge-prohibiting voltage, that is indicative of a voltage beyond which the battery will be overcharged, and turns the second switching element 32 into the conducting state when the battery voltage is above a discharge-prohibiting voltage, that is indicative of a voltage below which the battery will be over-discharged. In other words, when the rechargeable battery 30 complies with suitable charge/discharge conditions, the first and the second switching elements 31 and 32 are conducting, and the charge/discharge circuit for the rechargeable battery 30 establishes a conducting connection to the input/output terminals 34 and 35.
When the battery voltage is above the charge-prohibiting voltage, the charge/discharge circuit is interrupted, because the control means 33 turns the first switching element 31 into the interrupting state, and the charging is stopped, so that the rechargeable battery 30 is prevented from becoming overcharged. Conversely, when the battery voltage is below the discharge-prohibiting voltage, the charge/discharge circuit is interrupted, because the control means 33 turns the second switching element 32 into the interrupting state, and the discharging is stopped, so that the rechargeable battery 30 is prevented from becoming over-discharged.
As shown in FIG. 6, the first and second switching elements 31 and 32 both have parasitic diodes 31a and 32a between source and drain. The first and second switching elements 31 and 32 are connected in a manner that the forward direction of the parasitic diode 31a of the first switching element 31 corresponds to the discharge direction of the rechargeable battery 30, and the forward direction of the parasitic diode 32a of the second switching element 32 corresponds to the charge direction of the rechargeable battery 30.
Thus, when the control means 33 turns the first switching element 31 into the interrupting state to prevent overcharging, a discharging circuit is formed through the parasitic diode 31a, so that the rechargeable battery 30 can be discharged through the parasitic diode 31a, and when discharging has caused the battery voltage to drop below a charge-permitting voltage that is lower than the charge-prohibiting voltage, the control means 33 turns the first switching element 31 into the conducting state, and returns to a discharge state, in which the parasitic diode 31a cannot be passed. Conversely, when the control means 33 turns the second switching element 32 into the interrupting state to prevent over-discharging, a charging circuit is formed through the parasitic diode 32a, so that the rechargeable battery 30 can be charged through the parasitic diode 32a, and when charging has caused the battery voltage to rise above a discharge-permitting voltage that is higher than the discharge-prohibiting voltage, the control means 33 turns the second switching element 32 into the conducting state, and returns to a charge state, in which the parasitic diode 32a cannot be passed.
By using the parasitic diodes 31a and 32a of the first and second switching elements 31 and 32 in this fashion, discharging is possible when charging is prohibited, and charging is possible when discharging is prohibited, so that the rechargeable battery 30 can still be used when the circuit is interrupted to prevent overcharging and over-discharging of the rechargeable battery 30.
However, in this conventional configuration, there is the problem that during the time until a voltage is reached at which the charge-prohibiting state or the discharge-prohibiting state is cancelled, discharging or charging is carried out through the parasitic diodes 31a, 32a, whereby, when a large current flows through the parasitic diodes 31a, 32a, heat is generated in the first switching element 31 or the second switching element 32, which may lead to their deterioration. That is, the power loss when a certain current flows through the parasitic diodes 31a, 32a in forward direction is several times higher than the power loss when the same current flows between drain and source in the power MOSFETs used for the first and the second switching elements. Even though the heat generated by the current flowing between drain and source does not cause a failure of the power MOSFETs, the same current may lead to thermal destruction and failure of the power MOSFETs when flowing through the parasitic diodes.
In the case where a plurality of cells are connected in series to constitute the rechargeable battery 30, it is not sufficient to detect only the battery voltage. That is, if there are variations in the battery capacity between various cells, those that have lower battery capacity may fall into a state of over-discharge. In an extreme case, some cells that have lower battery capacity may discharge until their battery capacity is zero, they will be charged by other cells, leading to rapid deterioration of the cells. Therefore, when the rechargeable battery is constituted by connecting a plurality of cells in series, control for preventing over-discharge must be effected in consideration of the overall balance of the battery capacity of various cells.